The invention relates to radio telecommunications systems. A subscriber of a radio telecommunications system, i.e. a subscriber station, such as a mobile phone or some other means of communication, may be registered in a radio network or radio system through control or traffic channels maintained by the base stations of the radio network.
Apart from system channels of radio telecommunications systems, direct mode channels can also be used, i.e. direct mode operation is applied. Subscriber stations using direct mode operation do not communicate directly with a radio network or its base stations. Direct mode channels are frequencies at which the mobile phones or other means of communication are able to communicate directly with one another without the system.
Direct mode channels are typically used in situations where, e.g., a group of hand-held mobile phones communicate with one another at such a long distance from the base station that system channels cannot be used.
Another important use of direct mode channels is in the addition of capacity when traffic increases rapidly in one part of the service area of the system, e.g. at one point of the radio network.
A direct mode channel is also called a direct or simplex channel, or a simplex connection. A direct mode channel is a channel that is typically not used by the system at all. It may be, for example, a channel with the same channel spacing as the channels of the system, e.g. 12.5 kHz or 25 kHz. Further, especially a direct mode channel used for repetition may be a pair of frequencies comprising uplink and downlink frequencies, or in a time division multiple access system (TDMA), uplink and downlink channels. Of the mobile phones operating on the direct mode channel, the transmitting station has then tuned its transmitter to the channel and transmits speech or data information. The other mobile phones set to direct mode operation have tuned their receivers to the same channel.
On a direct mode channel, both analog modulation and digital modulation can be used. A mobile phone transmitting on the channel can also send signalling information, such as information about access rights and priorities or a group operating on the channel, or data traffic. On a direct mode channel, messages can be encrypted, or signals can be transmitted in clear form.
Subscriber stations operating in direct mode communicate with other subscriber stations on a direct mode channel, without necessarily being in direct contact with the base stations of the radio network. Subscriber stations operating in direct mode can, however, communicate with the radio network through repeater stations. A repeater station is an equipment in which two radio units are interconnected. A repeater station typically comprises two transceivers, one of which operates in the downlink direction and the other in the uplink direction. A repeater station transmits the information messages sent on the direct mode channel to the desired network elements, such as the base stations of the radio network, which forward the information messages to the mobile exchanges of the radio network. The information messages transmitted by the repeater station may be, e.g., data or signalling messages. With a repeater, e.g. semiduplex communication is used on a direct mode channel.
A problem with the solutions of the prior art is how to implement a reliable mobile communication system comprising a repeater that is cheaper to use and does not involve great equipment costs.
Another problem with the prior art solutions is how to provide a repeater and an operating method by which a repeater that is as simple as possible operates effectively, repeating and relaying information between the base station of the mobile communication system and the mobile stations that communicate or have been located on a direct mode channel.